Method for Thermally Treating Polyester Pellets to Obtain a Partial Crystallization

ABSTRACT

The invention relates to a method for thermally treating polyester pellets to obtain a partial crystallization, whereby the polyester melt is delivered to an underwater granulating system and granulated; the thus obtained granulates are carried from the underwater granulating system through a water-solids separator over a short conveying distance; the dried granulates are then delivered to a treatment device at a granulate temperature higher than 100 degrees C. without external energy or heat being supplied. The heat treatment leading to partial crystallization results from the intrinsic heat of the granulates, whereby the treatment device is embodied as an at least lightly inclined reactor, into which the granulates are delivered at a temperature higher than 100 degrees C., the granulates passing through said reactor from the loading point to the discharge point under their own weight and leaving the same at a temperature higher than 130 degrees C.

The invention relates to a method of thermally treating polyester pellets, to obtain a partial crystallization according to the preamble of the main claim, and apparatus for achieving same in accordance with the preamble of claim 3.

DESCRIPTION OF THE PRIOR ART

Aromatic polyesters or co-polyesters, particularly polyethylene terephthalate and its co-polymers, as well as polytrimethylene terephthalate and polyethylene naphthalate, formed into granulates, are the base material for products such as foils, packaging, as well as barrels or containers. All of these polyesters and co-polyesters shall be designated generally as “polyesters” in the description of the present invention.

A conventional method for thermally treating polyester pellets is disclosed in the subsequently published Application Publication WO 2005/044901 A1 (BKG). This method teaches very quickly removing water from the pellets that have just been produced, for example, in an “underwater heat-rejection granulation system,” by using the intrinsic heat of the pellets for drying and crystallization. The pellets are transported over a vibrating conveyor immediately after the water is removed, in order to prevent the pellets from sticking to each other and, after sufficient dwell time, delivered to a filling station or a solid-phase poly condensator. With this method, the crystallization process occurs from the inside to the outside of the pellet, which achieves a more even crystallization across the diameter of the pellet or granulate. Thus, this method achieves crystallization exclusively by using the intrinsic heat generated in the liquid state of the polymer. This means that no externally supplied heat is applied to the pellets between granulation and the subsequent filling station or post-processing station. This distinguishes the method according to WO 2005/044901 A1 basically from conventional methods.

A method of manufacturing polyethylene terephthalate granulates is described in the publication GB 1 250 690 A. According to this method, the base material is produced as in the conventional liquid condensation and delivered to an underwater granulator as an approximately 280 degree C. hot polyester liquid. The strand of hot synthetic material that is extruded through the jet is captured and cooled by a water spray coming out of a ring jet. This strand is then delivered through an underwater cooling distance to a cutting device. After passing through the cutting device, the granulate material traverses a sieve, by means of which water is separated from solid material. The water is pumped across a cooler in a closed circuit. The wet granulate material, after being dried, can be used for thermal post-condensation. This granulate material, particularly when treated in a thermal post-condensation at greater than 200 degree C., is suitable in its solid state for injection molding.

Essential with this process is that the product that is produced in the strand granulation process, i.e., an underwater cold-rejection granulating system, is inevitably a substantially cooled product, and has to be re-heated again, if it is to undergo any further thermal treatment, including processing for partial crystallization. This requires a substantial amount of energy, which energy, i.e., heat, is lost with the preceding cooling method.

U.S. Pat. No. 5,540,868 teaches how to produce crystallized pellets from amorphous polyester, using various granulating methods. The amorphous polyester pre-product has to be heated to temperatures greater than 70 degrees C., in order to initiate the crystallization process. Amorphous polyester, however, has a disadvantage in that, when heated to temperatures greater than 70 degrees C., its surface becomes sticky. In order to prevent the amorphous polyester from sticking or forming clumps at crystallization temperatures of greater than 70 degrees C., the pre-product has to be in the form of granulates, which can then be held in motion in a fluidized bed reactor by means of streams of hot gas, at least until the surface has crystallized out enough, so that the pre-product granulates do not stick to each other.

Amorphous polyester is transparent; the crystalline phase, however, clearly shows a white coloration. Normally, the crystallization process of the pre-product is combined with the additional reinforced poly condensation, which is typically carried out at temperatures between 200 and 230 degrees C. in a fluidized bed reactor. This is done to overcome the stickiness of the amorphous polyester. To achieve this, the reactor is operated first at an optimal crystallization temperature of approximately 150 degrees C. for several hours, to overcome the stickiness. Subsequently, the pellets or granulates are condensed over additional hours to higher chain lengths at temperatures between 200 and 230 degrees C.

The method according to the invention is not described in the subsequently published German Application Publication DE 10 2004 050 356 A1. In contrast, rather, claim 11 recites a method that includes a heat-insulating container in the post-processing granulating apparatus.

Language is provided, shown below, at the end of paragraph 0010 on page 3 that describes which method step is to be performed with this heat-insulating container.

-   -   “The hot granulate material can also be stored in a         heat-retaining condition, for example, in a heat-insulating         container, in order to complete the desired crystallization         process.”

Thus, the granulate material is not transported through the heat-insulating container, but rather, stored in the container, with the hope that this storage will lead to crystallization of the granulates, without the granulates baking and sticking to each other. Furthermore, the granulates are to be held in a heat-retaining condition, that is, they are to not cool down or heat up.

Aside from the fact that storing hot granulates that are exiting the dryer, without a motion device or without a motion of the granulate material itself will only result in the granulates sticking to each other, the teaching of this reference does not teach the method of the present application.

An object of the invention of the present application is to improve the conventional method, according to which partial crystallization is achieved by the intrinsic heat of the pellets from inside to the outside, so that once again a further simplification of the process is achieved.

According to the invention, the inventor suggests constructing the treatment apparatus, which is defined in the WIPO Publication WO 2005/0044901 A1 as motion equipment, as a crystallization reactor that is oriented at least slightly inclined or vertical. According to the inventive method, the pellets, having a temperature of greater than 100 degrees C., are loaded into the reactor, pass through this reactor from the loading point to delivery point, and exit this crystallization area having a temperature greater than 130 degrees C.

“Slightly inclined” means, that the reactor does not stand or lie horizontally, but rather, has an incline relative the horizontal of up to 90 degrees.

It was a surprising discovery to realize, that the exothermal process within the reactor prevents a sticking or clumping of the pellets. This is the case even though the pellets are relatively densely packed inside the reactor and pass through the reactor only by force of their own weight. The dwell time of the granulates in the reactor lies between greater than one minute and, preferably, eight minutes. The partially crystallized granulates are then delivered to downstream treatment or handling apparatus, for example, to a solid-phase poly condensator, a silo, etc.

The invention relates to apparatus for carrying out the method. The apparatus comprises a crystallization reactor tube, through which the pellets, based on their own weight, flow from the loading point to the delivery point. The reactor tube is operable with or without baffles. The baffles have the effect of guiding the granulates and thereby resulting in a blending or homogenization of the crystallization process. To this end, in accordance with a further embodiment of the invention, a rotating paddle-bearing shaft may be provided within the reactor, which effects a thorough mixing of the pellets.

Preferably, the reactor is heat-insulated and the granulates are transported between the exit end of the reactor and the connecting downstream post-processing apparatus or a silo, for example, by means of a blower that is incorporated into the conveyor tube.

The granulates pass through the reactor densely packed, that is, the volume of granulate material taken from the bottom is continuously being replaced from above. Furthermore, the volume of the granulate material being removed is also adjustable.

An embodiment of the invention will now be described with reference to the drawing.

FIG. 1 designates with 1 a melting pump and a sieve exchanger. Polyester is fed to the receiving apparatus 1, as indicated by arrow F1. An underwater heat-rejection granulator 2 is provided at the exit of the sieve exchanger, where bead-shaped or lentil-shaped granulates are formed. These granulates are delivered by means of process water via a conveyor tube to a water-solids separator 3, such as, for example, a centrifuge. The granulates then leave the water-solids separator 3 by means of a conveyor tube 4, the granulates having a temperature greater than 100 degrees C. The granulates are delivered to a crystallization reactor 5, which is shown in the illustrated embodiment as a vertically oriented reactor. A reactor that is at least slightly inclined may also be provided in a similar manner. The granulates pass through the reactor 5 from the loading point or the loading end to the exit or discharge end. A conveyor device 6 is provided at the discharge end, as shown in FIG. 1. The conveyor device 6 transports the granulates to a solid-phase poly condensator 8 by some means, such as, for example, a blower.

FIG. 2 also shows a vertically oriented crystallization reactor 5A, which has a shaft 9 fitted with paddles 10. The shaft 9 is rotated by means of a drive motor 11. This reactor 5A, too, may also be set up with a slight incline.

In both cases, the reactor 5, 5A is provided with heat insulation, although this is not absolutely necessary.

Temperatures and process times are provided in the FIG. 1, whereby the following designations are used:

PW is temperature of the process water; GT is the temperature of the granulate material; and VZ is the process time.

As shown, the temperature of the granulate as it is delivered to the underwater heat-rejection granulator is greater than 230 degrees C. The temperature of the process water in the conveyor tube between the granulator 2 and the water-solids separator 3 is greater than 80 degrees C. The granulates leave the conveyor 4 with a temperature greater than 100 degrees C.

The process time of the granulates in the reactor 5 is greater than one minute and is, preferably, eight minutes.

The temperature of the granulates entering the reactor 5 is greater than 100 degrees C., and is higher than 130 degrees C. at the discharge end of the reactor 5, without energy being supplied, in order to, for example, subsequently temper the granulates.

The temperature of the granulates when entering the solid-phase poly condensator 8 is also preferably greater than 130 degrees C.

These temperatures represent the surface temperature of the granulates.

In one embodiment the crystallization reactor 5 had a height of 1 m, an inner diameter of 270 mm, which narrowed to 100 mm at the discharge or delivery end. This exit opening may also be adjusted in size. The reactor 5 stood vertically, that is, was vertically oriented. 

1: Method for thermal treatment of polyester pellets, to obtain a partial crystallization, wherein the polyester melt is fed to an underwater granulation system and is granulated, the thus obtained granulates traverse a short conveyor distance from the underwater granulation system to a water-solids separator, the dried granulate material is then delivered to a treatment apparatus, without external energy or heat being supplied, the granulates having a temperature of greater than 100 degree C., and wherein heat treatment that results in the partial crystallization is achieved by means of the intrinsic heat present in the granulate, characterized in that the granulates are fed to a crystallization reactor that is at least slightly inclined, the granulates having a temperature of greater than 100 degrees C., the granulates passing through the crystallization reactor from the loading point to discharge point and then leaving the crystallization area with a temperature of greater than 130 degrees C., without energy being supplied. 2: (canceled) 3: Device for carrying out a method of thermal treatment of polyester pellets, in order to achieve a partial crystallization of the pellets with an underwater granulator (2) and a water-solids separator (3), characterized in that a reactor tube (5, 5A), being oriented at least slightly inclined, a conveyor device (6) being arranged at the lower end of the reactor tube, the conveyor transporting the partially crystallized granulates to a silo, a solid-phase poly condensator (8), or to other post-processing apparatus. 4-7. (canceled) 